Switched digital video (SDV) refers to an arrangement in which broadcast channels are only switched onto the network when they are requested by one or more subscribers, thereby allowing system operators to save bandwidth over their distribution network. In conventional cable or satellite broadcast systems, every broadcast channel is always available to all authorized subscribers. In contrast, a switched digital video channel is only available when requested by one or more authorized subscribers. Also, unlike video on-demand, which switches a singlecast interactive program to a user, switched digital video switches broadcast streams, making each stream available to one or more subscribers who simply join the broadcast stream just as they would with normal broadcast services. That is, once a switched service is streamed to a subscriber, subsequent subscribers associated with the same service group as the first subscriber can tune to the same broadcast stream. The switched digital video will often share the same resource managers and underlying resources with other on demand services.
As noted, switched digital video is largely a tool to save bandwidth. From the subscriber perspective, he or she still receives the same broadcast video service when using a switched broadcast technique; ideally the user is not able to discern that the stream was switched at all. If each one of the digital broadcast channels is being watched by subscribers in the same service group, the switched digital video approach does not yield any bandwidth savings. However, a more likely situation statistically is that only a certain number of the digital broadcast channels are being watched by subscribers in the same service group at any given time. Those channels not requested by a subscriber need not be broadcast, thereby saving bandwidth.
One way to support switched digital video is to utilize the session manager to manage SDV and other sessions. For each channel change, the subscriber will set up a broadcast session with the session manager, which will determine if the requested channel is already being sent to the corresponding service group that the subscriber belongs to.
The subscriber will be assigned to join the existing SDV session if the requested channel is available at the service group or assigned to a new SDV session if the requested channel is not available at the service group. The session manager will negotiate with the edge devices to allocate resources required for the session. The edge device (e.g., a digital modulator such as a QAM modulator) needs to dynamically retrieve the MPEG single program transport stream that carries the requested SDV program (likely via IP multicast or possibly IP unicast) and generate the MPEG multiple program transport stream. As part of the session setup response message, the video tuning parameters such as frequency and MPEG program number are sent back to the subscriber to access the requested SDV channel.
Each time the subscriber requests an SDV program the subscriber terminal sends a channel change request to the session manager. The channel change requests include the subscriber's service group number so that the session manager can properly identify the subscriber terminal and the edge device(s) with which it is associated. Accordingly, the subscriber terminal must be able to determine the service group to which it belongs. The subscriber terminal determines its service group using an autodiscovery process, which is performed the first time the subscriber requests an SDV channel and periodically thereafter. The autodiscovery process involves two phases. In the first phase, the subscriber terminal sequentially tunes to frequencies in a frequency plan that is specified in a configuration message which has been previously provided by the session manager. The subscriber terminal will continue to search the frequencies until it discovers the particular frequency or frequencies on which the service group number is made available. The second phase of the autodiscovery process involves, in one instance, extracting the service group number from the appropriate frequency or frequencies.
The autodiscovery process can occupy a significant amount of time because the subscriber terminal may need to search a relatively large number of frequencies before the service group number is discovered. This can cause an undue delay between the time the subscriber requests an SDV channel and the time the subscriber receives the SDV channel, thereby inconveniencing the subscriber.